Metal/GaN contacts studied by electron spectroscopies
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While the difference χm - χs between the electronegativity of the metal and of the semiconductor is specific to each metal, the slope parameter Sχ and the charge neutrality level Φcnl are characteristic of GaN. They were calculated by Kampen and Mönch [1] who obtained Sχ = 0.29 eV/Miedema-unit. and Φcnl = 2.35 eV with respect to the top of the valence band. Bermudez [2] investigated the dependence of the structure and electronic properties of wurtzite GaN surfaces on the method of preparation. Ordered and unreconstructed surfaces were obtained. A bare surface barrier height was measured on these surfaces that depends on the annealing temperature and reaches 0.7eV for surfaces annealed at 700°C. When Mg was grown on wurtzite GaN surfaces, the energy difference between the Fermi level and the valence band maximum EF-VBM increased with Mg coverage while no intermixing between Mg and GaN was observed [3]. Mohney and Lin [4] used a thermodynamical approach to predict the phase diagrams of many transition metal-Ga-N systems and to explain the electrical properties of the contacts.The structure and bonding at the metal/GaN interface are expected to affect electronic properties of the contact and in particular its barrier height and specific resistance. The present paper describes a compositional analysis of Au/GaN and Cu/GaN interface formation and the evolution of the Schottky barrier height during the interface formation. These studies are based on in situ XPS measurements.
F99W11.79
EXPERIMENTAL PROCEDURE GaN epitaxial layers were grown at CRHEA (Valbonne, France) on (0001) sapphire substrates by metalorganic vapor phase epitaxy (MOVPE) at atmospheric pressure[5]. A 100 Å thick AlN buffer layer was used. Non intentionally doped samples showed n-type conduction, with a carrier concentration of about 1017 cm-3, and a Hall mobility of about 130 cm2/V.s. These samples had to be cleaned prior to metallization. Several cleaning procedures will be discussed below. They usually involve wet chemical etching, followed by various treatments in ultra-high vacuum (UHV), such as annealing, exposure to atomic hydrogen, or exposure to a Ga flux. The atomic H was obtained from an RF plasma source (Oxford Applied Instruments, Model MPD20) operated at 175W RF power. The base pressure in the preparation chamber was 5 10-10 Torr. The plasma was ignited with nitrogen instead of hydrogen. The nitrogen was then gradually replaced by hydrogen. A flux of 9 sccm H2 (purity N57), rising the pressure to 2 10-2 Torr, gave the brightest plasma (checked with a photodiode). During these treatments, samples were heated to 700°C. Ga was evaporated from a conventional effusion cell heated to 850°C. The Ga layer thickness, estimated from the XPS signals, ranged from 20 to 60Å. Au and Cu were deposited by thermal evaporation from effusion cells equipped with BN crucibles. Background pressure was about 2 10-10 Torr prior to evaporation, and rose to 1 10-9 Torr during the metal deposition. Au was evaporated in a UHV chamber pumped by an ion
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